I didn't read all the answers above, so apoligies for duplicate info.
wheel stiffness depends almost entirely on the total amount of steel, namely the cross section X the no. of spokes.
Shorter spokes also increase stiffness, but the difference isn't great because the difference between the shortest (radial) and longest spokes possible is very small.
Rim lateral stiffness is also a factor, especially if spoke counts are lower, because stiffer rims spread the load more, bringing more spokes into play.
Then other factors come into play, including flange separation and spoke hole offset in rims, which affect the bracing angles & whether the spokes are woven over/under at the cross.
However TENSION HAS NO EFFECT ON STIFFNESS.
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